Lithography and offset printing methods have long been combined in a compatible arrangement of great convenience for the printing industry to provide economical, high speed, and high quality image duplication in various run sizes. Known lithographic methods have generally required the imaging of a photosensitive film followed by exposing a lithographic printing plate precursor using the film as a masking element, and development of the resulting printing plate image with various aqueous developers and rinsing solutions.
In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Alternatively, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer usually includes one or more radiation-sensitive components that can be dispersed or dissolved in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask that has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque mask regions. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
While this process has been used for many years to provide high-quality images, the process is relatively inefficient and laborious. More recently, advances in the industry have provided image formation by digital computer aided design of graphical material or text. This manner of “computer-to-plate” (CTP) imaging is extremely advantageous because images can be easily edited or converted prior to imaging. This imaging method is particularly useful for the lower run printing jobs.
Thus, direct digital imaging has obviated the need for imaging through a mask and is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, multi-layer elements are described, for example, U.S. Pat. No. 6,294,311 (Shimazu et al.), U.S. Pat. No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al.), and U.S. Pat. No. 6,528,228 (Savariar-Hauck et al.), U.S. Patent Application Publication 2004/0067432 A1 (Kitson et al.).
Other digital imaging methods have been achieved using lasers as described for example in U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), and U.S. Pat. No. 5,351,617 (Williams et al.) wherein the laser output ablates one or more layers of the imageable element, resulting in an imagewise pattern of features on the element. Producing images by ablation has its disadvantages including unwanted debris and vapors from the imaging process and images that may lack desired sharpness because imaged areas are incompletely removed. Ablated matter requires appropriate removal from the imaging layer and disposal.
Digital imaging can also be achieved by laser exposure of a heat-sensitive layer in an imageable element whereby imaged areas are selectively changed in affinity for aqueous solutions or oleophilic inks, depending upon the composition of the heat-sensitive layer. Examples of this type of imaging are provided, for example, in U.S. Pat. No. 5,985,514 (Zheng et al.), U.S. Pat. No. 6,159,657 (Fleming et al.), U.S. Pat. No. 6,190,830 (Leon et al.), U.S. Pat. No. 6,190,831 (Leon et al.), U.S. Pat. No. 6,399,268 (Fleming et al.), U.S. Pat. No. 6,410,202 (Fleming et al.), U.S. Pat. No. 6,447,978 (Leon et al.), and U.S. Pat. No. 6,451,500 (Leon et al.).
Positive-working lithographic printing plate precursors containing an IR-sensitive polythiophene, polypyrrole, or polyaniline in a single imaging layer are described in U.S. Pat. No. 5,908,705 (Nguyen et al.). These imaging layers are designed to provide a positive-working image by removal of the imaged areas of the IR-sensitive polymer by ablation. The IR-sensitive polymers are formed either by solution polymerization prior to coating or by in-situ polymerization on the substrate after the appropriate monomers are deposited on the substrate by vapor deposition or coating and contacted with a suitable catalyst. If solution polymerization is used, it is preferred to crosslink the IR-sensitive polymer with a polymeric binder. U.S. Pat. No. 5,908,705 specifically discourages the use of solid “preformed” polypyrroles or other IR-sensitive polymers because it results in non-uniform distribution of the polymer throughout the coating, and hence, non-uniform ablation.
Huang and Kaner in Nature Materials, Vol. 31 (November), 2004, Nature Publishing Group, describe nanofibers of polyaniline that are welded into a composite using flash irradiation. Various speculative uses of the nanofibers are proposed but none is demonstrated.